This invention relates to an apparatus for feeding a powdery material to a plasticized polymer mass. Such powdery material usually contains entrapped air, and it has been a serious problem to feed and to mix the powdery material with the main polymer feed. When the two are introduced together in the main feed hopper of an extruder, the entrapped air is forced into the extruder itself and, as pressure within the extruder increases, the air is forced rearwardly, out of the main feed opening, entraining the powdery material and propelling it rearwardly out of the entrance, thus interfering with the entire feeding operation and causing a safety and housekeeping problem in the plant in which the extruder is located.